Hollow filaments have found wide application in heat transfer, mass transfer and separations. In the separation processes it is common practice to apply a pressurized fluid mixture to the external surface of the hollow filaments made from a semi-permeable material having the propensity for permitting the selective permeation of one or more components of the mixture through the filament wall into its bore more rapidly than some other. The more permeable component(s) therefore are enriched in the bore relative to the composition applied under pressure to the external surface of the filament. The residue of the pressurized fluid remaining on the upstream surface of the filament membrane, consequently, is enriched in the less permeable component(s). The contents of the bore are removed from the module separately from the residue of pressurized feed fluid flowing along the external surface of the filaments. This process has become increasingly important for the separation of air into its principal components, oxygen and nitrogen.
In commercially feasible applications a relatively large surface area of membrane is required in any installation. Thus, practical use of hollow filament membranes requires assembling a large number of them into a unit, frequently called a bundle, which is then assembled into a module with appropriate conduits to pass a feed mixture over the external surfaces of the filaments, to remove permeate from the filament bores, and to remove feed residue.
An alternative arrangement for the use of hollow filaments provides means to introduce pressurized feed into the filament bores, so that permeation of the more permeable components is from the bore to the outside surface of the filament. The arrangement of feed, permeate and residue conduits is, therefore, different from that of the case of "shell-side" feed. Although the flow dynamics of bore-feed operation are different from that of shell-side feed, for either case there is a decided advantage in having a filament bundle of regulated and essentially uniform packing density, adequate distribution of inter-filament spaces and generally uniform filament length.
In shell-side feed, particularly, flow patterns of the feed, residue, and permeate are very important in determining the efficacy of the separation sought. It is widely recognized that geometry of the bundle assembly has a significant influence on these. It is desired that the feed gas on the outer surface of the filaments be in true counterflow mode against the permeate flow in the filament bores. It is also desired that the path lengths within the bores be essentially uniform, and the local flow velocities on the shell side be both relatively high and uniform from point to point within any plane intersecting the bundle at right angles to its long axis.
These several considerations dictate certain geometric requirements for the bundle which thereby place high demands on the bundle assembly process. First, variability of filament-to-filament packing density would desirably be nil. Second, to maintain a relatively high shell-side flow velocity, the packing density should optimally be quite high, namely in the order of 50-70%. Third, feed flow along the outside surface of the filament should be directed as much as possible parallel to the length of the filament whether the filament is straight or follows some curved path, which generally requires that the aspect ratio (bundle length to bundle diameter) be at least three and as much as ten. Fourth, however, there is the requirement that bore path length not introduce undesirable restriction on flow and consequent undesirable permeate pressure.
In attempting to meet these objectives the configurations of hollow filament bundles has heretofore taken a number of different forms including (a) arrays of parallel, straight or nearly-straight lengths of filament; (b) arrays of grossly parallel lengths of crimped filament; (c) lengths of active filament interwoven with inactive filaments; (d) filaments braided into groups; (e) arrays of filament formed into helices wherein an equal number with an "s"-direction helical path interlace with filaments following a "z"-direction helical path; and (f) helical filament arrays comprising discrete layers wherein parallel filaments all lying in "s"-direction helices adjoin filaments of an adjacent layer all lying in "z"-direction helical paths and in which there is no interlacing of filaments of opposite helix direction.
The first two arrangements generally suffer from at least some, and sometimes considerable variation in local filament packing density. The result of this is that flow along the outside of the filaments can be high in some regions on effectively stagnant in others with consequent deleterious effect on the fluid dynamic requirements for optimum separation performance. The third and fourth methods tend to provide more uniform packing density but lead to complexities and high costs in the assembly process.
The last two general approaches are partially responsive to packing density considerations but introduce undesirable flow perturbations. They also may suffer in respect to filament length uniformity unless special attention is paid to control of the helix angle of the filaments. They are, however, appealing from the manufacturing point/of view and exploited as illustrated in U.S. Pat. Nos. 3,794,468; 3,870,637; 4,045,851; 4,430,219; 4,572,446; and 4,631,128. In all these cases the filaments are wound on a support element constantly rotating in one direction as filaments are laid down on the support in a continuous path which traverses the support from one of its end to the other. The combination of mandrel rotation and filament end-to-end traversal results in accumulating on the support element lengths of filament all lying in helical paths. Because the rotation continues in one direction when the filament traverse direction is reversed, the length of filament deposited on any traverse from a first end to a second end lies in an "s-"direction helix, and in a "z-"direction when the traverse is from the second end back to the first.